In formation of a heterojunction structure formed of nitride-based semiconductors such as AlGaN layer/GaN layer, these layers have been conventionally formed through epitaxial crystal growth onto a sapphire substrate or a Si substrate because a GaN substrate is expensive.
Regarding crystal growth of a nitride-based semiconductor layer on a Si substrate, various buffer layer structures have been contemplated to relax a strain that is based on a crystal structure difference of the nitride-based semiconductor layer from the Si substrate, lattice mismatch therebetween, a thermal expansion coefficient difference therebetween, and the like. More specifically, there are many patent publications in relation to a superlattice buffer layer structure including repetition of two types of superlattice constituting layers.
For example, in order to form a nitride semiconductor layer, which has a smooth surface and includes no crack, on a substrate greatly different therefrom in terms of lattice constant and thermal expansion coefficient, Patent Document 1 describes that an AlGaN-based superlattice buffer layer structure is formed by repeatedly and alternately providing first superlattice constituting layers of AlxGa1−xN (0.5≦x≦1) and second superlattice constituting layers of AlyGa1−yN (0.01≦y≦0.2) on the silicon substrate.
On the other hand, in order to suppress current leakage between electrodes via a buffer layer, Patent Document 2 describes a semiconductor element having: an AlN buffer layer on a silicon substrate; and a superlattice buffer layer structure including H—AlGaN superlattice constituting layers of high Al composition ratio and L-AlGaN superlattice constituting layers of low Al composition ratio, which are alternately provided thereon.
Moreover, in Patent Document 3, a composition-graded buffer layer structure is formed on a Si substrate or an intermediate layer formed thereon, a superlattice buffer layer structure is then formed thereon, and then a nitride layer to serve as a channel layer is formed thereon. The composition-graded buffer layer structure has an Al composition ratio continuously or stepwisely decreased in the thickness direction. The superlattice buffer layer structure includes AlGaN superlattice constituting layers of high Al composition ratio and AlGaN superlattice constituting layers of low Al composition ratio, which are alternately provided on each other. It is described that by employing such a lamination structure, there is obtained a semiconductor element having a nitride semiconductor layer that is less likely to have crack and pit generated therein and that has excellent crystallinity.